Snap acting device



Feb. 8, 1955 Filed April 1'7, 1950 R. M. READEKER SNAP ACTING DEVICE 2 Sheets-Sheet l MEMMLW Feb. 8, 1955 R. M. READEKER 2,701,475

SNAP ACTING DEVICE OVERHANG (1N.)

WITHOUT HOLE United States Patent Q SNAP ACTING DEVICE Robert M. Readeker, Freeport, 111., assignor, by mesne assignments, to Minneapolis-Honeywell Regulator Company, Minneapolis, Minn., a corporation of Delaware and more particularly to an improvement in a snap acting spring system of the type employed in precision snap acting electric switches of a well known type.

While prior art snap acting spring systems employing springs of the general type herein contemplated, when properly constructed, may commonly be operated for several millions of operations without failure of the members forming the spring: system, some applications require a snap acting spring construction having a still greater life expectancy than has heretofore been possible. When switches of this character ultimately fail, it is sometimes due to fracture of the tension member near the edge of the anchor. Springs of this character are commonly actuated by flexing the tension member about its cantilever mounting to etfect snap motion and one plausible explanation for such failure is that the flexing of the tension member necessary to effect actuation of the spring system is such as to cause it to fail by fatigue of the metal.

An object 'of this invention is the provisionof a snap acting spring system of the above character having a greatly increased life expectancy over comparable spring systems.

Another object of the invention is the provision of a snap acting spring system having novel means for distributing stresses in the tension member, during actuation of the spring system, so as to minimize fracturing of the tension member.

Another object :of the invention is the provision of a snap acting spring system of the. above character having a relatively long life that may be produced at a relatively low cost.

Other objects and advantages of the invention will become apparent from the following detailed description taken in connection with. the accompanying drawings, in which-- Figure 1 is' a longitudinal sectional view of a snap acting switch embodying the present invention;

Fig. 2 is an enlarged top viewof the snap spring system showing the snap acting spring and an anchor plate for mounting the same, and

Fig. 3 is a graph showing the contour of a conventional spring system and the contour of the spring system embodying the present invention at difierent operating positions of the spring systems.

Referring now to the drawings the invention is shown embodied in an electric switch. For purposes of illustration the latter includes a snap acting spring system 11, mounted in a housing 13 of suitable insulating material, and a plunger 14 mounted on the housing to actuate the snap spring system 11. The snap spring system 11 in this embodiment of the invention is formed by spaced compression members 16, atension member 17, means for mounting the tension and compression members to stress them respectively in tension and compression and opposed's'tops 18 and 19 for limiting movement of the tension member between opposed positions.

To mount the compression members 16 one end of each compression member is pivotally supported in grooves 22 formed at the ends of legs 23 of a U-shap ed plate 24 which is mounted on a shoulder 26 formed at one end of the housing 13 opposite the stops 18 and 19 and secured by a screw 37 threaded intoa sleeve 38 disposed in the housing 13. The opposite ends of the compression members are interconnected with an end of the tension member to define a free'end39 spaced from F ce Patented Feb. 8, 1955 the mounted ends of the compression member and shaped to carry a movable contact 41 between the opposed stops 18 and 19. As best seen in Figure 1 the tension member 17 is disposed between the compression members 16 and is mounted in a cantilever manner as by attaching the mounted end of the tension member between a crosspiece 42 of the anchor plate 24 and the screw 37. .The central portion of the crosspiece 42 is offset relative to the plane of the legs 23 to form a recess for the mounted end of the tension member. A screw 43 threaded into the sleeve 38 secures a terminal 44 on the exterior of the housing 13.

The stops 18 and 19 in this embodiment of the invention are defined by fixed contacts which are formed by spaced metal elements secured to shoulders 46' and 47 respectively at one end of the housing as by screws 48 and 49. The screws are threaded into conducting sleeves such as sleeve 51, molded in the housing 13 and electri: cally connected to a terminal 52 on the exterior of the housing. The terminal 52 is secured to the sleeve as by a screw 53. Where desired, one of the contacts may be-omitted and an insulating member substituted there- I of the housing 13, and a head 57 having a projection 57a shaped to engage the tension member 17 at a point between the cantilever mounting of the tension member and the pivotal supports for the compression members 16.

One phase of the invention is concerned with distributing stresses in the tension member 17 so as to increase the life of the spring system and reduce the liklihood of fracturing the tension member as a result of flexing the latter member. .For this purpose the tension member 17 is formed with a reduced cross sectional area at a point adjacent the cantilever mounted end to increase the flexibility or reduce the rigidity of a portion of the tension member so as to control the curvature of the tension member .and prevent the formation of highly stressed areas immediately adjacent the cantilever mounted end of the tension member. While there are a number of ways of reducing the cross sectional area of the tension member, in this embodiment of the invention the tension member 17 has a portion intermediate its edges removed to define a generally rounded aperture 58 and side portions 59 interconnecting the mounted end and the opposite end portions of the tension member. The-aperture 58 is located between the point of overhang of the tension member 17 at its cantilever mounted end and the point of application of the plunger 14 and it is thought that the best results are obtained when the center of the aperture is at the point of inflection of the tension member, determined for the tension member in the absence of the aperture 58, thepoint of inflection of the tension member being the point wherein the curvature of the tension member changes from a convex curve to a concave curve during actuation of the spring system when viewed along a longitudinal section of the tension member (see Fig. 3). -Theconfiguration of the aperture may be circular, elliptical or other shape; In some instances a plurality of apertures may be utilized and the side edges ofthe tension member may have portions removed to reduce the cross sectional area of the tension member. For optimum results sharp corners in the configuration of any aperture or any relieved portion should be avoided. While the tension member 17 in the present embodiment of the invention is a single member, it is to be understood that where there are a plurality of spaced tension members, each of them may be formed with apertures or have portions of the sides removed to define portions having greater flexibility than the remainder of the members as described above.

When the plunger 14 is depressed the tension stress line is moved through a plane or axis of maximum stress defined by the juncture between the tension member 17 and the compression members 16 and the pivotally' supoperating positions, i. e., the free position, the position at which the spring system goes overcenter and the .015 overtravel position. The solid line curves show the configuration of a tension member 17 having an aperture 58 located as described hereinbefore while the dotted line curves show the configuration of a tension member without an aperture or reduced cross section for purposes of comparison. In general, it will be seen from these curves in Fig. 3 that as the plunger 14 is depressed, the overhanging portion of the tension member 17 between the plunger 14 and the cantilever mounted end of the tension member assumes, as seen in viewing Fig. 3, what can best be described as a generally reversing curve or S-shaped curve, with the overhanging portion of the tension member immediately adjacent the cantilever mounted end having a convex configuration and the portion of the tension member adjacent the point of application of the plunger having a concave confi uration. Observation of the curves shows that the tension member 17 with an aperture 58 requires a greater plunger movement to effect actuation of the spring system than is the case of the tension member without the aperture. Thus, greater flexure of the tension member 17 takes place in the spring system having the aperture than in a spring system without the aperture. Normally it would be expected that the spring having the greatest flexure would have the shortest life. Tests and experience. however, show that spring systems having the aperture 58 have a very substantially longer life than spring systems without an aperture in the tension member. Normally it would also be expected that a tension member 17 having an aperture 58 therein and subjected to bending stresses would have greater bending in the area of the aperture than at other portions of the tension member. An inspection of the curves shown in Fig. 3 shows however that there is no discontinuity of curvature in the tension member in the area of the aperture 58 and it appears that the aperture is in a position where substantially zero bending effect occurs.

From the aforegoing it'is seen that the spring system having a tension member 17 with an aperture 58 opcrates in a manner that is entirely contrary to normal expectations. One explanation of the phenomenon is that in the tension member 17 with the aperture 58 the bending moments are changed so that the bending stresses are distributed rather uniformly over the area of the tension member between the cantilever mounted end and the aperture 58. This is in contrast to a spring system having a tension member without an aperture wherein the portion of the tension member immediately adjacent the point of overhang appears to be an area of concentrated stresses during actuation of the spring system. The configuration of the tension member 17 with the aperture 58 for different operating positions of the spring system substantiates the view that the bending stresses are redistributed. Thus, it can be seen in Fig. 3 that when the aperture is located at the point of inflection as described hereinbefore, a longer portion of the tension member 17 adjacent the cantilever mounted end is convexly curved (see solid line curve) than is the case of the tension member of a spring system without an aperture (see dotted line curve). Also it will be noted that the curvature of the tension member 17 with the aperture is more arched and not as abrupt as in the case of a tension member without an aperture. Thus, by-

increasing the flexibility of an intermediate portion of the tension member 17 the bending stress can be distributed so that the curvature of the tension member can be controlled to prevent the formation of areas of concentrated stress immediately adjacent the point of overhang. As a result the life of a spring system utilizing a tension member with an aperture is greatly increased as compared to a spring system utilizing a tension member without an aperture.

Inasmuch as there is no apparent bending of the tension member 17 in the portion thereof having the aperture. itis not apparent why the relatively small bending that does occur would so alter the stresses as to produce the magnitude of effect apparent in the new construction, and it seems probable that distribution of the bending stresses is not the sole cause for the improved results.

It has been suggested that the results are due to a combination of factors including distributitu of the wave stresses commonly present in such spring systems coupled with distribution of the cantilever stresses. High speed motion pictures of spring performance and photographic analysis of contact bounce indicate the presence of a complex wave motion in the spring system after its operation. This wave motion, in following standing wave theory, must necessarily be reflected from the point of cantilever mounting of the tension member. It is well known that a point of wave reflection is a high stress area and it is entirely possible that reflection of wave motion is one of the reasons for the concentrated stress adjacent the mounting in spring systems of this character. In a spring system in which the tension member has an aperture 58 it would appear that waves traveling from the free end of the spring system toward the cantilever function at the portions 59 of the tension member in such a manner as to cause such portions to appear as the source of new sets of waves and that the interference pattern between the two sets of waves causes energy dissipation to be distributed over the portion of the spring between the aperture 58 and the cantilever mounting rather than be concentrated directly at the cantilever mounting.

There is also undoubtedly partial wave reflection from theaperture 58 toward the free end 39 and since the periphery or margin of the aperture 58 is not perpendicular to the axis of the spring, differential portions of the waves at selected positions across the width of the spring will be reflected at times mutually out of phase. This will again produce energy dissipation by wave interference, and also shield the. portion of the tension member 17 immediately adjacent the cantilever mounting from the stresses which occur in the spring without the aperture.

I claim:

1. In combination, a member having opposite end portions, means for mounting one of said end portions to provide a cantilever mountingfor said member, means for supporting the other end of said member, means for applying a force intermediate the ends of said member to flex the latter to conform to an S-shaped configuration adjacent the cantilever mounted end and means on said member for increasing the flexibility of said member in the area of the point which in the absence of said means on said member would be the point of inflection of said member, whereby to redistribute the stresses in said member to preclude fracturing thereof immediately adjacent said cantilever mounted end due to flexing said member.

2. In combination, an elongated resilient member having opposite ends, means for mounting one of said ends to provide a cantilever mounting for said member, means for supporting the other end of said member, a plunger for applying an actuating force intermediate the ends of said member to flex the latter to have the portion adjacent said cantilever mounting means conform to an S-shaped configuration. said member having a reduced cross section at a portion of its extent spaced from its point of overhang at its cantilever mounted end and located between said point of overhang and the point of application of the plunger to increase the flexibility of the member therebetween to distribute the bending stresses in said member over a substantial portion thereof immediately adjacent its cantilever mounted end as said actuating force is applied to the member.

3. In combination, an elongated resilient member hav-' ing opposite'ends. means for mounting 'one of said ends to provide a cantilever mounting for'said member, means for supporting the other end *ofgis'aid. member, a plunger for applying an actuating force intermediate the ends of said member to flex the latter;. t o. have the portion adjacent said cantilever mounting' means conform to an S- shaped configuration, said elongated member having an aperture located to have its center coincide with the point which in the absence of said aperture would be the point of inflection of said member to increase the flexibility of said member and distribute the stresses over a substantial portion of the member during flexing thereof.

4. In a snap action device, the combination of a ten sion member having an end free to move between opposed positions with a snap action, a compression member interccnnected at one of its ends with said free end, means for supporting the opposite end of the tension member to provide a cantilever mounting therefor, means for mounting the compression member to define with said tension member a snap acting spring system having an axis of maximum stress, means for moving said tension member through an axis of maximum stress and means formed on said tension member in spaced relation from its point of overhang at its cantilever mounted end and located between said point of overhang and said last mentioned means adjacent the point which in the absence of said means formed on the tension member would be the point of inflection of said tension member for distributing the stresses throughout a substantial portion of the tension member adjacent the cantilever mounted end thereof and thereby minimize the concentration of stresses immediately adjacent the cantilever mounting means.

5. In a snap acting device, the combination of a tension member having an end free to move between opposed positions with a snap action, spaced compression members interconnected at one of their ends with said tension member, means for supporting the opposite end of the tension member to provide a cantilever mounting therefor, means for mounting the opposite end of each compression member to define with said tension member a snap acting spring system having an axis of maximum stress and a plunger for moving said tension member through said axis of maximum stress, said tension member having a portion of reduced cross sectional area at a position spaced from its point of overhang at its cantilever mounted end and located between said point of overhang and the point at which it is engaged by said plunger immediately adjacent the point which in the absence of said reduction in cross sectional area would be the point of inflection ofsaid tension member toincrease the flexibility of a portion of said tension member between the cantilever'mounted end and the point at which it is engaged by the plunger to control the curvature of the tension member adjacent the cantilever mounted end and thereby minimize the stresses in the tension member immediately adjacent the cantilever mounting means when the tension member. is flexed.

6. In a snap acting device, the combination of a ten sion member having an end free to move between opposed positions with a snap action, spaced compression members interconnected atone of their ends with said free end, means for mounting the opposite end of the compression members to define with said tension member a snap acting spring system'having an axis of maximum stress and a plunger for moving said tension member through said axis of maximum stress, said tension member having an aperture located to have its center coincide with the point which in the absence of said aperture would be the point of inflection of said tension member to increase the flexibility of the tension member intermerdiate the cantilever mounted end of the tension member and the plunger to'distribute the stresses throughout a substantial portion of the tension member and thereby minimize high stresses immediately adjacent the cantilever mounted end of the tension member when the latter is flexed to actuate the spring system.

7. In a snap acting mechanism, the combination of a tension member having an end free to move between opposed positions, means for mounting the opposite end of the tension member in cantilever fashion, a compression member interconneced at one end with said free end, means for supporting the opposite end of the compression member to define with said tension member a snap acting spring system having an axis of maximum stress, means for moving a portion of said tension member through the axis of maximum stress to move said free end with snap action between said opposed positions, and means on the tension member spaced from its point of overhang and positioned intermediate said moving means and said point of overhang of the tension member to damp out vibration.

and a hole in said first member spaced from its point of overhang at its cantilever mounted end and positioned between the point of application of the plunger and said point of overhang to damp out vibration.

9. In combination, an elongated resilient member having opposite ends, means for mounting one of said ends to provide a cantilever mounting for said member, means engaging said member in spaced relation from its cantilever mounted end stressing said member in tension, a. plunger for applying an actuating force intermediate the ends of said members to flex the latter to have the portion adjacent said cantilever mounting means conform to an S-shaped configuration, said member having a reduced cross-section at a portion of its extent spaced from its point of overhang at its cantilever mounted end and located between said point of overhang and the point of application of the plunger to increase the flexibility of the member therebetween to distribute the bending stresses in said member over a substantial portion thereof immediately adjacent its cantilever mounted end as said actuating force is applied to the member.

' 10. In combination, an elongated resilient member having opposite ends, means for mounting one of said ends to provide a cantilever mounting for said member, means engaging said member in spaced relation from its cantilever mounted end stressing said member in tension, 2. plunger for applying an actuating force intermediate the ends of said member to flex the latter to have the portion adjacent said cantilever mounting means conform to an S-shaped configuration, said elongated member having an aperture located to have its center coincide with the point which in the absence of said aperture would be the point of inflection of said member to increase the flexibility of said member and distributes the stresses over a substantial portion of the member during flexing thereof.

References Cited in the file of this patent UNITED STATES PATENTS Re. 19,300 Eggleston et al. Sept. 4,, 1934 1,960,020 McGall May 22, 19% 2,181,068 Riche Nov. 21, 1939 2,340,615 Roth Feb. 1, 1946 2,349,008 Sauter May 16, 1944 2,361,202 Hodgins Oct. 24, 1944- 2,469,686 Eaton May 10, 1949 

